alleryStart learning →A long wire carries current I through a uniform field B at an angle theta. What is the magnetic force per unit length of the wire?
AF/l = IB sin(theta)
BF/l = IB cos(theta)
CF/l = I/B sin(theta)
DF/l = B/I sin(theta)
Answer & Solution
Correct answer: A. F/l = IB sin(theta)
1. Start from the total force on a length l of wire, F = IlB sin(theta).
2. Divide both sides by the length l.
3. This gives F/l = IB sin(theta) for the force per unit length.
4. Writing the result per metre is convenient when the wire is very long and no single length applies.
5. Dividing by B rather than multiplying would make a stronger field give a weaker force, which is wrong.
_Source: OpenStax College Physics (CC BY 4.0), Ch 22 "Magnetism", section 22.7 Magnetic Force on a Current-Carrying Conductor_
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